1. Field of the Invention
The present invention relates to engine control and more particularly to the combustion control of compression ignition engines, such as diesel engines or CAI (Controlled Auto-Ignition) engines.
2. Description of the Prior Art
Operation of a diesel engine is based on auto-ignition of a mixture of air, burnt gas and fuel. The engine cycle can be broken down into several phases (FIG. 1):                upon intake (ADM), intake valve (SADM) allows the mixture of air and of burnt gas into chamber (CHB). The air is taken from the outside environment of the engine. The burnt gas is taken from exhaust manifold (ECH) and sent back to the intake manifold (exhaust gas recirculation EGR). This gas mixture fills combustion chamber (CHB) and mixes with the burnt gas remaining in the chamber since the previous combustion (internal EGR);        the intake valve closes (IVC: Intake Valve Closing). Piston (PIS) compresses the gas;        fuel injector (INJ) injects a precise mass of fuel. After a short auto-ignition delay period, the mixture of air, burnt gas and fuel ignites, thus creating an overpressure that drives the piston downwards;        once the piston has been driven down again, exhaust valve (SECH) opens and the gas mixture is thus discharged through the exhaust manifold. After closing of the exhaust valve, part of the gases remains in the cylinder (internal EGR). The gases discharged through the exhaust manifold are divided in two. A part is recirculated towards the intake (EGR) whereas the rest is discharged out of the engine (via the exhaust).        
The goal of such engine control is to supply the driver with the torque required while minimizing the noise and pollutant emissions. Control of the proportions of different gases and of fuel therefore has to be adjusted as finely as possible.
To carry out combustion control of a compression ignition engine, there are known methods allowing determination of the combustion medium by detectors mounted in the engine. The most precise means uses a pressure detector in the combustion chamber. Such a method is described for example in the following document:                J. Bengtsson, P. Strandh, R. Johansson, P. Tunestal and B. Johansson, “Control of Homogeneous Charge Compression Ignition (HCCI) Engine Dynamics”, Proceeding of the 2004 American Control Conference, Boston, Jun. 30-Jul. 2, 2004.        
However, using such detectors in standard vehicles cannot be considered due to the considerable cost of such detectors. Furthermore, these detectors are generally subject to relatively fastdrifting.
There are also methods wherein the proportions and timing are optimized on each static working point (speed and torque) so as to bring out an ideal strategy at each point. A test bench calibration is therefore performed in order to obtain the optimum values for the main two data sets:                the mass of air Mair and of burnt gas Mgb required in the combustion chamber, denoted by Xair=(Mair,Mgb);        the mass of fuel Mf and the crank angle θf at which the fuel is injected, denoted by Xfuel=(Mf,θf).        
However, these strategies are insufficient in transient phases. In fact, during transition phases from one working point to another (change in the vehicle speed or in the road profile), the engine control supervises the various actuators present in the engine to guarantee the desired torque while minimizing the noise, the pollutant emissions and the consumption. This is thus translated into the change from the values of the parameters of the initial point to the values of the parameters of the final point:
         {                                                      X              air              initial                        →                          X              air              final                                                            (            a            )                                                                          X              fuel              initial                        →                          X              fuel              final                                                            (            b            )                              
Now, there are two time scales in the engine. The faster one (50 Hz) corresponds to the entire combustion phenomenon (1 engine cycle). On this scale, the injection strategy (Xfuel) can be changed to control the combustion. It is the fuel loop (see (b)). The slower one (1 Hz) corresponds to the gas dynamics in the engine manifolds (intake, exhaust, burnt gas recirculation). The strategy of this air loop (Xair) cannot be changed faster (see (a)).
With the current methods, the controlled variables (Xair, Xfuel) do therefore not reach at the same time their setpoint values because of this difference in dynamics. The objectives regarding torque production, consumption, pollutants, noise are thus met in the static phases (the two dynamic loops are stabilized at their reference values); on the other hand, if precautions are not taken in the transient phases, part of the parameters nearly instantaneously reach their final setpoint value whereas the other part is still at the initial setpoint values, which causes the engine to produce more pollutant emissions or noise, and it can even stop in some cases.
Furthermore, without cylinder pressure detectors, the known methods do not allow control of combustion timing during the transient phases. Now, as illustrated by FIGS. 2 and 3, this is not sufficient to ensure proper operation of the engine under transient conditions.